The invention relates to an optical reader for reading an image on a document or paper.
A conventional optical reader of this kind comprises a plurality of light emitting elements which illuminate or irradiate a plurality of reading points on a document or paper through a plurality of optical fibers, a plurality of light receiving elements which receive reflected lights from the plurality of points on the paper through a plurality of optical fibers, and read means which reads an image on the paper on the basis of the relative magnitudes of outputs from the plurality of light receiving elements as compared against a single given reference value. Such optical reader reads an image on the paper by determining if a particular image on the paper represents a white or black relative to the single reference value. Accordingly, when an image on the paper is depicted in multiple halftones, the optical reader mentioned above has been incapable of reading the image on the paper in halftones.
Recently, it is desired by users that an optical reader of the kind described be capable of reading an image or images on a paper in multiple halftones. At this end, the read means is supplied with a plurality of reference values. As an image on the paper is read on the basis of a relative magnitude with respect to a greater one of the plurality of reference values, the image read by the read means is determined as representing an intermediate color which is close to the white color. If an image on the paper is read by a comparison against a smaller one of the plurality of reference values, the image read by the read means is determined as an intermediate color which is close to the black color. When an arrangement is made such that such optical reader reads adjacent spots on the paper in the pattern of a dither matrix, for example, using a plurality of reference values, an image on the paper can be read in multiple halftones.
There arises a problem, however, that the plurality of light emitting elements which are contained in the optical reader may have non-uniform light emission due to variation in their operating characteristics or variation due to aging effect. If the plurality of light emitting elements produce an equal amount of light, outputs from the plurality of light receiving elements may still be non-uniform if the connection between the light emitting elements and the associated optical fibers or between the light receiving elements and their associated optical fibers are not perfectly in close adherence. Like the light emitting elements, the light receiving elements may produce non-uniform outputs because of variations in the operating characteristic among them and also due to the aging effect. Accordingly, it becomes necessary that an individual one of the plurality of reference values which are set up in the read means be adjusted to a proper magnitude in accordance with an output value from a particular light receiving element. When the number of reference values which must be adjusted increases, the resulting adjustment is both time and labor consuming operation.